Spark gap unit for lightning arresters



. D66. 3, 1968 11R CONNELL ET AL 3,414,759

SPARK GAP UNIT FOR LIGHTNING ARRESTERS 5 Sheets-Sheet 1 I Filed Dec. 1,1966 I INVENTOR L w L N Mr m 3 n P A RE sr MN AR Dec. 3, 1968 T RCQNNELL ET AL 3,414,759

SPARK GAP UNIT FOR LIGHTNING ARRESTERS 3 Sheets-Sheet 2 Filed Dec. 1,1966 INVENT OR THOMAS R. CONNELL ROBERT E. PUTT 1'- lLLlHlllllhl FIG. 3

ATTORNEY Dec. 3, 1968 T, R, CQNNELL ET AL. 3,414,759

SPARK GAP UNIT FOR LIGHTNING ARRESTERS Filed Dec. 1, .l96 5 Sheets-Sheet5 FIG. /0

INVENT OR THOMAS R. CONNELL ROBERT E. PUTT H648 m MM ATTORNEY UnitedStates Patent 3,414,759 SPARK GAP UNIT FOR LIGHTNING ARRESTERS Thomas R.Connell, Wadsworth, and Robert E. Putt,

Clinton, Ohio, assignors to The Ohio Brass Company,

Mansfield, Ohio, a corporation of New Jersey Filed Dec. 1, 1966, Ser.No. 598,467 11 Claims. (Cl. 315-36) ABSTRACT OF THE DISCLOSURE A sparkgap unit for lightning arresters with a cen tral gap plate and an endplate on each side thereof defining spark chambers for two seriesconnected spark gaps. It provides the functions and results of the priorart many-plate gap units with improved manufacturing procedures.

This invention relates to spark gaps for lightning arresters, voltagedischarge devices, are switches, and the like.

A principal object of the invention is to simplify manufacture of sparkgaps embodying stacked assemblies of ceramic gap plates.

Another object of the invention is to increase the mechanical strengthof spark gaps and ceramic gap plate arrangements and to facilitate theuse of porous ceramic materials in gap plates.

Another object of the invention is to increase the arc voltage andelectrical strength of arc gaps for lightning arresters and the like.

In spark gaps for lighting arresters and the like, it is desirable toutilize large numbers of relatively small gaps to achieve the necessaryvoltage and current capability for a given arrester. A large number ofparts, all of the same size and construction, may be manufactured andthen associated in assemblies having electrical characteristicsdetermined by the number of gap elements in the assembly. Whileproviding many advantages, this type of spark gap presents certaindesign problems inasmuch as it is difiicult to associate the gapelements in a determinate way to produce a mechanically strongassemblage of arrester elements which can be provided with an insulatedhousing in a compact space. This is an important requirement since thearrester elements are arranged in stacks and columns and the variousfunctions must be provided at an economically acceptable cost.

According to the present invention, three ceramic gap plates areassociated to form two gap chambers and to provide an enclosure for twopairs of gap electrodes. One of the gap plates, referred to as thecenter plate, is formed on the opposite sides thereof to cooperate withthe remaining two plates, referred to as the end plates, and theadjacent surfaces of the center plate and end plates are spaced apart toform two separate arc chambers. The gap electrodes are positioned sothat the arc gap for initial sparking is near the center of the gapplate, and the two pairs of electrodes are rotated through 180 degreesin a modified helical arrangement in accordance with the teachings ofUS. Patent 3,019,367, issued Jan. 30, 1962. Assemblies of gap plates arearranged in stacks with the associated magnetic coils and valve blocks,as described in the patent.

The invention, togethef with further objects, features, and advantagesthereof, will be understood from the following detailed specificationand claims taken in connection with the appended drawings in which:

FIG. 1 is a side elevation view, in section, of a lightning arresterembodying the spark gap of the instant invention;

3,414,759 Patented Dec. 3, 1968 FIG. 2 is a schematic diagram of theelectrical circuit of the lightning arrester of FIG. 1 and theassociated conductor and ground with which it is used;

FIG. 3 is a side elevation view of the spark gap of the invention;

FIG. 4 is a top plan view of the spark gap, taken in the direction 4-4in FIG. 3;

FIG. 5 is a top plan view of the bottom gap plate of the spark gap ofFIG. 3, taken along the line 5--5 in FIG. 3;

FIG. 6 is a section view of the bottom gap plate, taken along the line6-6 in FIG. 5;

FIG. 7 is a section view of a gap plate assembly, taken in the direction7-7 in FIG. 4;

FIG. 8 is a section view of a gap plate assembly, taken along the line88 in FIG. 4;

FIG. 9 is a view of a center gap plate, taken along the line 99 in FIG.3; and

FIG. 10 is a view of the opposite side of the center gap plate of FIG.9, taken along the line 10-10 in FIG. 3.

Referring now to FIG. 1, the lightning arrester 10 embodying theinvention comprises an elongated housing 11 of porcelain provided on,the outside with a plurality of skirts 12 for increasing the leakagedistance along the outside of the housing, and formed on the interiorwith an opening 13 longitudinally through the housing for receiving theoperative elements of the arrester. The ends of the housing 11 areclosed by formed metal members 14 and 15 which engage the housing memberalong peripheral grooves 16 and 17 adjacent the end of the housing. Thejoints between the members 14 and 15 and the end faces of the housing 11are sealed by means such as O-rings 18 and 19 for maintaining theinterior of the housing separate from the exterior thereof. The endparts 14 and 15 are designed to enclose a stack of operative arresterelements 20 in the opening 13 of the housing, and the entire assembly iscarried on end pieces (not shown herein) which are cemented over theends of the housing and over the end pieces 14 and 15.

The operative elements of the arrester are arranged in a stackcomprising two piles of non-linear resistors or valve blocks 21 and 22and a plurality of spark gaps or gap assemblies 23, 24, 25, and 26, eachassembly including a magnetic coil unit, a plurality of gap plates, andgrading devices, as hereinafter described. The valve blocks and gapassemblies are electrically connected by mechanical contact between theadjacent elements by metal plates which are attached to the elements atthe opposed faces thereof, the arrester elements being maintained incompressive relation with each other and with the end members 14 and 15by means such as a spring 27.

FIG. 2 shows the circuit arrangement of the lightning arrester 10connected between an electrical conductor 30 and a ground 31 accordingto its intended use for discharging overvoltages from the conductor 30to the ground 31 in a protective circuit 32. The actual protectivecircuit includes the entire arrester 10 while the circuit 32 of FIG. 2illustrates only that portion of the operative elements of the arrester10 which includes the gap assembly 26 and valve blocks 22. The remainingones of the gap assemblies 23, 24, and 25 are the same as the gapassembly 26, and the valve blocks 21 are the same generally speaking, asthe valve blocks 22.

The spark gap apparatus 33, representing the gap assembly 26, comprisesfour main gaps 34, 35, 36, and 37 connected in series and having certainassociated gaps, resistors, and capacitors connected in parallel toconstitute four gap control stages, referred to generally as S1, S2, S3,and S4, all connected in series with a coil 38,

3 associated with the gaps 34 to 37. The valve blocks 22 arerepresentative as a valve resistor 39.

The coil 38 comprises :a means for generating a magnetic field toelongate the arcs and the several main gaps 34 to 37. Two fixed gaps 40and 41 are connected in shunt with the coil 38 and function duringinitial dis charge of energy from the line 30 to the ground 31, throughthe gaps 34 to 37. After a suflicient time interval, current flow in thecoil 38 extinguishes the arcs in the gaps 40 and 41 so that anincreasing magetic field is generated by the coil for moving the arcs inthe gaps 34 to 37. The .gaps 40 and 41 may be replaced by arrangementsof non-linear resistors, as is described in U.S. Patents 2,825,008 and3,019,367 to J. W. Kalb.

In the apparatus 33, the gap control stage S1 is constituted by a maingap 34, a switching gap 42 having series connected switching resistors43 and 44, and a grading resistor 45. Gap control stage S2 comprises amain gap 35, a switching gap 46 with series switching resistors 47 and48, and a grading resistor 49. The gap control stage S3 comprises a maingap 36, a grading capacitor 50, and a grading resistor 51. The gapcontrol stage S4 comprises a main gap 37, a switching gap 52 with seriesresistors 53 and 54, and a grading resistor 55. The described parts areconnected in parallel circuits to constitute the designated stages.

The stages S1, S2, and S4 have the same grading resistances, that is,the grading resistors 45, 49, and 55 are equal, whereas the resistanceof the grading resistor 51 of stage S3 is small relative to resistors45, 49, and 55. The switching gaps 42, 46, and 52 are designed andadjusted to spark over at substantially the same voltage, less than thebreakdown voltage of the main gaps, but the associated series resistors,that is, the resistors 43 and 44 of the gap 42, the resistors 47 and 48of the gap 46, and the resistors 53 and 54 of the gap 52, areprogressively smaller for the successive stages so that an unequalvoltage division exists as between the main gaps 34, 35, and 37 afterthe'switching gaps have fired.

The switching gaps 42, 46, and 52 are small gaps having preciselydimensioned electrodes positioned adjacent the main gaps 34, 35, and 37.The switching gaps function as preionizers for the main gaps as well asa switching means for the several switching stages, and are adapted fordischarge in a narrow, predetermined range of voltages over the range ofdesign tolerances. In production, one of the gaps 42, 46, :and 52 isselected to spark over at a voltage close to the design voltage, andminor production variations in the remaining switching gaps do notmaterially alter the overall functioning, inasmuch as the gap 42determines the firing sequence of stages because of the larger values ofresistors 43 and 44.

In operation, the line voltages are divided between the several maingaps 34 to 37 in the same proportions as the resistance of theindividual grading resistors 45, 49, 51, and 55 to the total gradingresistance constituted by the four resistors. Accordingly, with thegrading resistances heretofore described and, prior to discharge of theswitching gap 42, the line voltage is divided with equal voltages acrossstages S1, S2, and S4 and a much lesser voltage across S3.

Upon the occurrence of an overvoltage on line conductor 30, theswitching gaps 42, 46, and 52 spark over and connect the switchingresistors 43 and 44 in parallel with the grading resistor 45, theswitching resistors 47 and 48 in parallel with the resistor 49, and theswitching resistors 53 and 54 in parallel with the grading resistor 55.The resistors 43 and 44 are substantially larger than the resistors 47and 48, which :are in turn substantially larger than the resistors 53and 54. Accordingly, after the switching gaps 42, 46, and 52 have fired,the distribution of line voltage as between the stages S1, S2, and S4 isaltered so that a greater proportion of the line voltage appears acrossthe main gaps 34, 35, and 37, in that order, in accordance with theinequality between the parallel resistances of the stages S1, S2, andS4. Accordingly, the gap 34 fires first, followed by the gap and the gap37, whereupon all the line voltage appears across the gap 36 which isfired last. This completes the firing sequence and initiates thedischarge sequence in which transient energy is discharged from the lineconductor 30 to the ground 31, through the valve resistor 39. In thedischarge sequence, the coil 38 functions to elongate the, arcs of themain gaps 34 to 37 to terminaate the discharge, all as described in U.S.Patents 2,825,008 and 3,019,367 heretofore referred to.

'Ilhe functioning of the spark gap apparatus just described occurs whenthe lightning arrester is subjected to line overvoltages which haveslowly rising wave fronts, or switching surges, so called. For lineovervoltages which have rapidly rising wave fronts, or impulseovervoltages, so called, the grading capacitor functions to distributethe line voltage between the gaps 34, 35, and 37 so that those gaps arefired, whereupon the line voltage is impressed across the gap 36 whichis fired last. The construction and functioning of the gap controlcircuit, to gether with other circuit arrangements useful in thepractice of the present invention, is described in detail in co-pendingU.S. application, Ser. No. 585,846, filed Oct. 11, 1966.

The gap assemblies 23, 24, and 25 function in the same way as the gapassembly 26 just described. The several assemblies functionsimultaneously because of the series connections of the circuits andbecause of grading resistors, such as the resistors 56 and 57 connectedacross 1 the gap assemblies 23 and 26, for distributing the impressedvoltage substantially equally between the several gap assemblies.

"The gap assembly 26 is shown in an enlarged exterior View in FIG. 3. Asthere shown, a plurality of end gap plates 60, 61, 62, and 63 cooperatewith associated center gap plates 64 and 65 to enclose and define sparkgaps, associated arc chambers, and control and discharge circuits of therespective stages S1 to S4 of FIG. 2. The end plates 60 to 63 engage theassociated center plates 64 and. 65, along peripheral ridge and recessmeans and interengage the end faces of a plate member 66 which carriesthe coil 38 at the center position of the six gap plates. The assemblyis arranged in the form of a stack and the elements are connected inseries with two end plates 67 and 68 which connect the gap assembly tothe adjacent operative elements of the stack 20 or to the terminals ofthe arrester housing.

The invention will be understood by reference particularly to FIG. 5 andFIGS. 7 to 10 inclusive of the drawing illustrating the construction ofthe gap plates 62, 63, and 65. FIG. 5 illustrates the gap plate 63 whichis substantially identical with gap plates 60 and 61 and identical withthe gap plate 62 except in the substitution, in that plate, of thecapacitor 50 for a switch gap and switching resistors in the plate 63.

In FIG. 5 the gap plate 63 comprises a circular ceramic plate having acircumferential ridge 69 with the interior surface 70 of the platedefining an arc chamber 71. Two main gap electrodes 72 and 73,comprising the main 1 gap 37, are carried by the plate 63 and the plate65,

respectively, as is hereinafter described. Two elevated portions orridges 74 and 75, integral with the body of the plate 62, extend abovethe surface 70 intermediate between the level of that surface and thelevel of the ridge 69. The ridges 74 and 75 extend from adjacent theinterior of the electrodes 72 and 73, transversely of the gap plate tothe ridge 69, and define the circumferential extremities of the arcchamber71 and the limit of elongating arc movement from the adjoiningarcing faces of the electrodes 72 and 73, The ridge 69 extendscircumferentially only to the ridges 74 and 75, and a recess 76 isdisposed along the periphery of the plate 63 between the ridges 74 and75 for receiving a ridge on the gap plate 65, as is hereinafterdescribed.

A formed ceramic member 77 is received between the ridges 74 and 75 andis secured to a surface 78 between the ridges 74 and 75, disposedsomewhat below the level of the surface 70. The member 77 carries twoelectrodes 79 and 80 which constitute the switching gap 52 and theswitching resistors 53 and 54. The electrodes 79 and 80 are formed ascylindric bodies of resistive material which are received in recesses inradially extending portions of the member 77 and have the interior endsclosely spaced at a predetermined distance apart to constitute the gap52, the bodies of the electrodes constituting the series switchingresistors 53 and 54. The grading resistor 55 is carried at the extremityof the member 77, and the lead wires of the resistor 55 are connected tothe electrodes 79 and 80 and to the electrodes 72 and 73 to constitutethe parallel circuit of stage S4.

The gap plate 63 is formed on the interior side thereof, illustrated inFIG. 5, to cooperate with the adjacent side of the gap plate 65,illustrated in FIG. 10. As shown in FIG. 10. the gap plate 65 is formedas a body having 'a fiat interior surface 81 which cooperates with thesurface 70 of the plate 63 to form the gap chamber 71, a recess 82 fromthe surface 81 extending peripherally about the plate 65 for receivingthe ridge 69 of the plate 63, two recesses 83 and 84 for receiving theridges 74 and 75 of the plate 63, a surface 85 spaced from the surface78 of the plate 63 for receiving the member 77 and the associated parts,and a ridge 86 extending along the peripheral extremity of the plate 65,between the recesses 83 and 84, for engaging the recess 76 in the plate63. Very shallow recesses 87 and 88 are formed in the surface 81 of theplate 65 for receiving and positioning the electrodes 72 and 73 and theplates 63 and 65.

A surface 89 recessed from the surface 81 of the plate 65 cooperateswith a like surface 90 recessed from the surface 70 of the plate 63 toconstitute an arcing chamber about the adjacent portions of theelectrodes 72 and 73, and the recesses 89 and 90 converge to thesurfaces 81 and 71 along surfaces 91 and 92, respectively, to constitutean arc entrance chamber connecting the arcing chamber with the arcelongation chamber defined by the surfaces 70 and 81 extending radiallyoutward from the electrodes 72 and 73. The surfaces 70 and 81 areparallel and spaced apart throughout the extent of the arc elongationchamber, as set forth in co-pending application Ser. No. 624,375, filedMar, 20, 1967 and the patents previously referred to.

As will be evident from FIG. 7, FIG. 8, and FIG. 10, that portion of thegap chamber 71 just referred to as the arc elongation chamber extendsradially from the electrodes 72 and 73 throughout the circumferentialextent of the gap plates between the barrier ridges 74 and 75. Adjacentthe radial extremity of the plates, the gap chamber 71 is defined by asurface 93 constituting the interior surface of the recess 82 of theplate 65, and a surface 94 comprising the interior surface of the ridge69 of the plate 63. As shown in FIG. 7, the surfaces 93 and 94 extendgenerally in the longitudinal direction of the gap assembly 26 from theradial extremity of the surfaces 70 and 81, and converge together fromthe parallel spacing of the surfaces 70 and 81. The convergent surfaces93 and 94 define an arc extinguishing chamber of the gap chamber 71.

The arcing chamber defined by the surfaces 89 and 90, the arc entrancechamber defined by the surfaces 91 and 92, and the arc elongationchamber defined by the surfaces 70 and 81 correspond, respectively, tothe arc recess, arc entrance chamber, and arc extinguishing chamber ofthe spark gaps described in US. Patents 2,825,008 and 3,019,367 abovereferred to. Accordingly, it will be understood that the arc elongationchamber defined by the surfaces 70 and 81 performs the functionperformed by that portion of the spark gaps described as the arcextinguishing chamber in the subject patents and that the function ofthe arc extinguishing chamber defined by the surfaces 93 and 94 is inaddition thereto.

The gap stage S3 is constituted by the gap plate 62 and the gap plate65, which define the gap chamber 96 for the gap electrodes 97 and 98.The gap chamber 96 and electrodes 97 and 98 (FIG. 7 and FIG. 8) arerotated about the longitudinal axis of the gap assembly 26, through anangle of 180 degrees with respect to the gap electrodes 72 and 73 andare chamber 71. The relative disposition of the two surfaces of theplate 65 will be understood from FIG. 9 and FIG. 10, which arerepresentations taken by rotating the opposed faces of the plate 65about the lateral axis and into the plane of the paper. The electrodes97 and 98 constitute the gap 36, and the grading resistor 51 andcapacitor 50 are mounted on a member 99, similar to the member 77carried by the plate 63, in the positions of the electrodes 79 and andresistor 55, respectively, of the plate 63 and member 77. The gapchamber 96 is formed in the same way as the gap chamber 71, includingarcing chamber, arc entrance, arc elongation chamber defined by surfaces100 and 101, and an arc extinguishing chamber defined by surfaces 102and 103, of the plates 62 and '65 respectively.

According to the invention, and referring to FIG. 7, the gap electrodes72 and 73 are secured to the end plate 63, the gap electrodes 97 and 98are secured to the end plate 62, and the gap electrodes 73 and 98 aresecured together to hold the gap plates 62, 63, and 65. To this end, thegap electrodes 72 and 97 are secured to the metal end plates 68 and 104by metal rivets 105 and 106 extending from the end plates through thebodies of the gap plates 63 and 62 to the gap electrodes. The gapelectrodes 73 and 98 are secured together and to the center plate 65 bymeans of a rivet 107 extending through the electrodes and through thebody of the center gap plate 65.

In manufacturing the assembly comprising the gap plates 62, 65, and 63,the gap plates are first made by pressing particulate ceramic materialinto the shape and form shown. The formed gap plates are then fireduntil the desired degree of rigidity and porosity is achieved. The endgap plates 62 and 63 are prepared by coating the adjacent surfaces ofthe electrodes and gap plates with epoxy resin, and then assembled withthe metal end plates. The assembly is heated to a predeterminedtemperature to cure the resin and bond the metal parts to the ceramicgap plate. In the gap plate 63, for example, the pin 106 of the endplate 63 is then riveted into place to connect the gap electrode 72 tothe end plate '68 and to secure the electrode and end plate to the gapplate 63. The end gap plates are then tested to determine the breakdownvolt-age, and those plates having breakdown voltages close to the designvoltage are segregated for use in stage S1. The remaining gap plates,within a wider range of breakdown voltages, are used for stages S2, S3,and S4.

The assembly comprising the member 77 with associated electrodes 79 and80 and grading resistor 55, or grading capacitor for stage S3, isattached to the end gap plate by bonding the member 77 to the body ofthe end plate 63 by means of epoxy resin. The resin is cured and theleads from the electrodes 79 and 80 soldered to the electrodes 72 and73.

The end gap plates are then assembled with the center gap plate, theadjoining faces of the peripheral ridges and recesses 69 and 82 and 76and 86 being coated with epoxy resin to bond the gap plates together.The pin 107 is then inserted through the openings 108 and 109 in the endplates and the opening 110 in the center plate, and riveted into placeto secure the gap electrodes 73 and 98 to the center plate and hold theend plates against the center plate while the resin along the adjoiningsurfaces of the ridges and recesses hardens to form a bond between theparts and to seal the interior of the gap. Alternatively, the plates areheld in a suitable jig while the resin is cured, and the heads are thenformed on the pin 107 to make an electrical or an electrical andmechanical connection between the gap electrodes 73 and 98. The openings108 and 109 are then sealed with porcelain or other ceramic plugs andepoxy resin.

In another embodiment, the pin 107 may be formed of a cOntact or otherconductive material having suitable elastic properties and the headomitted.

The plate assembly or spark gap comprising the plates 62, 65, and 63 isthen combined with a suitable plate assembly, comprising the gap plates60, 64, and 61, and a coil assembly comprising plate 66 and coil 38 toconstitute the gap assembly 26.

The several gap assemblies 23 to 26 are arranged in stacked superimposedrelation in the housing 11, with the gradin resistor 57 connected to themetal end plates 67 and 68 of the assembly 26, and similar gradingresistors connected to the remaining gap assemblies. The gap assembliesare connected in series with valve blocks 21 and 22 to constitute theoperative elements of the lightning arrester 10, and all connected tothe end parts 14 and 15. The housing is then closed and sealed, andsuitable end parts comprising terminal and support members are cementedin place over the ends of the housing.

It will be understood that the gap assembly 26 has important advantagesin the construction of the lightning arrester of FIG. 1. First, thearrangement of the plate assembly with two end plates cooperatin with asingle center plate formed on opposite sides thereof to constitute anarc chamber with each of the associated end plates, provides a spark gapunit having a relatively short extent along the longitudinal axis of thestack in relation to the arc voltage generated by the gap and inrelation to the energy dissipating capability of the gap. Again, thethree-plate construction embodies relatively thick gap plates andpermits the use of materials of relatively low modular strength so thatthe gap plates may be constructed of porous ceramic materials. Theresultant structure utilizes the desirable properties of porousmaterials in arc voltage generation and are extinction and providesheight saving of importance in lightning arrester-s for very highvoltages.

Another advantage of the three-plate construction described herein isthat manufacture of the plates themselves is simplified and expedited.Thus, the end plates 62 and 63 are of identical construction so thatonly two kinds of dies are required, one for the end plates and one forthe center plates. Again, the modular arrangement of the parts, in whichthe end plates are formed with one fiat surface on one side and elevatedsurfaces or projections from a central surface on the other side, andthe center plate is formed with recesses on both sides, simplifies diedesign and facilitates molding and pressing of particulate ceramicmaterials.

It is to be understood that the foregoing description is not intended torestrict the scope of the invention and that various rearrangements ofthe parts and modifications of the design may be resorted to. Thefollowing claims are directed to combinations of elements which embodythe invention or inventions of this application.

We claim:

1. A spark gap for an overvoltage protective device comprising three gapplates of rigid ceramic material, including an integral center plate, afirst end plate on one side of the center plate, and a second end plateon the opposite side of the center plate and each cooperating therewith,a first pair of two gap electrodes arranged between the first end plateand the center plate having laterally spaced outwardly divergent arcingfaces, and the adjacent faces of the first end plate and center platebeing spaced apart to constitute an arc chamber for receiving an areformed between the two gap electrodes, a second pair of two gapelectrodes arranged between the second end plate and the center plate,the adjacent faces of the second end plate and center plate being spacedapart to constitute an arc chamber for receiving a second are formedbetween the two gap electrodes, means securing each pair of electrodesto the adjacent end plate, means extending through the center plate fromone electrode of one pair of electrodes to one electrode of theremaining pair of electrodes electrically connecting the said oneelectrode, and means securing the two end plates to the center plate.

2. A spark gap in accordance with claim 1, in which the last two namedmeans comprise a pin mechanically connecting the said one electrode.

3. A spark gap in accordance with claim 1, in which the last two namedmeans comprises a metal pin extending through the said one electrode andcenter plate, and heads on the pins engaged with the exterior surfacesof the said electrodes for compressively holding the electrodes againstthe center plate.

4. A spar-k gap in accordance with claim 3, in which there are openingsin the end gap plates aligned with the pin for receiving the pin duringassembly of the gap plates.

5. A spark gap in accordance with claim 1, in which the said twoelectrodes are each bonded to the associated end plate in predeterminedspaced relation for predetermining the sparkover voltage of the sparkgap after assembly thereof.

6. A sparkgap in accordance with claim 5, in which there is a metal endplate over the exterior surface of the ceramic end plate, and theremaining one electrode of each pair of electrodes is electricallyconnected to the metal end plate by a metal pin extending through theceramic plate.

7. A spark gap in accordance with claim 1, 'with interengaging ridge andrecess means extending along the peripheral extremities of the gapplates for closing the lateral extremities of the gap chambers, and thelastnamed means comprises means bonding the plates along the adjacentfaces of the ridge and recess means and closing the arc chambersthereof.

8. A spark gap for an o vervoltage protective device comprising threegap plates having the bodies thereof formed of rigid ceramic material,including an integral center plate, a first end plate on one side of thecenter plate, and a second end plate identically the same as the firstend plate on the opposite side of the center plate and each cooperatingtherewith, a first pair of two gap electrodes arranged between the firstend plate and the center plate having laterally spaced outwardlydivergent arcing faces, and the adjacent faces of the first end plateand center plate being spaced apart to constitute an arc chamber forreceiving an are formed between the two gap electrodes, 21 second pairof two gap electrodes arranged between the second end plate and thecenter plate, the adjacent faces of the second end plate and centerplate being spaced apart to constitute an arc chamber for receiving asecond are formed between the two gap electrodes, means securing eachpair of electrodes to the adjacent end plate, means on the end gapplates comprising n'dges formed of ceramic material integral with thebody of the plate projecting from the said adjacent face of the end gapplate, and means on the center plate formed as recesses in the body ofthe plate from the said adjacent faces of the center plate for receivingthe ridges of the end plates, and means bonding the plates along theadjacent faces of the ridge and recess means.

9. A spark gap in accordance with claim '8, in which the gap electrodes,are chamber, and ridge and recess means of the second end plate andcenter plate are rotated through degrees about a longitudinal axisthrough the plates with respect to the gap electrodes, are chamber, andridge and recess means of the first end plate and center plate.

10. A spark gap in accordance with claim 9, in which the gap electrodesare in contact with the center plate and the associated end plateadjacent the transverse cen- 9 10 tral portion of the plates for mutualreinforcement of the References Cited plates along the said longitudinalaxis thereof. UNITED STATES P 11. A spark gap in accordance with claim 8or claim 9, in which the electrodes are bonded to the end plates, the 307 114 1 19 3 Hicks 315 3 X two pair of electrodes are connected inseries by means 5 3,242 37 3 19 Schultz 315 3 extending through thecenter plate from one electrode of one pair of electrodes to oneelectrode of the remain- JAMES LAWRENCE P'lmary Exammer' ing pair ofelectrodes. C. R. CAMPBELL, Assistant Examiner.

